[Python] Writing Parquet with pyarrow.dataset still defaults to Parquet version 1.0

While we just bumped the default Parquet version we write from 2.4 to 2.6 (https://github.com/apache/arrow/issues/35746), and this was bumped from 1.0 to 2.4 a year ago (https://github.com/apache/arrow/pull/13280), in both cases we missed to update that default in the `ParquetFileWriteOptions` for writing with `pyarrow.dataset.write_dataset`